While it's rather technical, it will give you some very good guidance
about choosing exercise that will make your back stronger without
damaging it further. Someone in your position cannot simply choose a
machine - you must be aware of the right form/technique to use,
regardless of the exercise. Proper exercise selection _and_ proper
technique together are what you need.

Just my opinion, but I have suffered a debilitating back injury - you
can read more about my history and my training at the site below if you
wish.